P51

Over the past 10 years, the number of respiratory infections caused by the Mycobacterium avium complex (MAC) has increased considerably in Russia. The high level of drug resistance (both intrinsic and acquired) and lack of breakpoints for many antibiotics are major issues in treating of MAC infections. The aim of this study was to investigate the phenotypic drug susceptibility of MAC strains isolated from patients with mycobacteriosis in Northwestern Russia. In total, 192 slow-growing MAC strains (164 - M. avium, 28 - M. intracellulare) isolated in 2014-2020 were studied. Of the 164 M. avium strains, 116 were isolated from newly diagnosed patients, 48 from previously treated patients. Drug susceptibility testing was performed using Sensititre SLOMYCO panels (Thermo Fisher Scientific, USA). For clarithromycin (CLA), moxifloxacin (MXF), linezolid (LZD) and amikacin (AMI), CLSI breakpoints have been used to interpret MIC values. Of the four antibiotics, CLA was the most effective against both M. avium (67.1%; 110/164) and M. intracellulare (60.7%; 17/28) without significant difference between species (p>0.05). Overall, 57.3% M. avium and 57.1% M. intracellulare isolates were susceptible to LZD. For MXF, 26.8% M. avium and 14.3% M. intracellulare isolates were susceptible; for AMI - 57.3% M. avium and 53.5% M. intracellulare isolates were susceptible, respectively. The proportion of CLA-susceptible M. avium isolates was 20% higher in newly diagnosed patients compared to previously treated patients (p=0.013). In conclusion, the macrolides are the most effective in the therapy of MAC infections, however, CLA-resistant strains emerged significantly more frequently after treatment.